The present invention relates to a signal verification system and, more particularly, to a system and technique for providing an audio verification of signal acquisition and tracking in a Global Positioning System (GPS) receiver.
As is known in the art, the new generation Global Positioning System has been developed to enable the determination of a position with respect to a plurality of satellites in orbits encircling the earth. The GPS system is designed to aid in air, land, and sea navigation by enabling the calculation of a position within very accurate tolerances based solely on a plurality of signals received from selected ones of the satellites forming the system. The techniques used to calculate position, as well as a variety of other parameters related to position (velocity, altitude, acceleration, etc.) are well known. As a result, there have been proposed a variety of embodiments for enabling sophisticated navigation as well as more simplified embodiments for charting automotive vehicular movement with respect to map displays. In all cases, the receipt of properly correlated signals from the plurality of satellites is necessary to enable the computation of desired position and related parameters in the various applications.
In the GPS system, each of the individual satellites generates a carrier signal having modulated information which is transmitted by spread spectrum techniques, wherein each satellite transmission is in accordance with a particular pseudo-random code. In order to calculate position, a GPS receiver selectively acquires and tracks the signal from a plurality of the satellites. In a GPS receiver, the spread spectrum signal is normally first translated to an IF frequency and subsequently correlated by mixing with a local pseudorandom code indicative of that code of the selected satellite. The resulting output signal is then substantially a data modulated (CW) carrier signal having a Doppler component due to satellite movement. The modulated CW signal is subsequently mixed with a local carrier including the same Doppler to produce a baseband output representing only the data information. This information, along with code ranging information, is then employed with similar information from the other satellites to allow the calculation of position.
In a properly operating GPS system, the acquisition will be accomplished with automatic techniques and the calculation of position will occur in accordance with the properly correlated and received satellite signals. Naturally, if the satellite signal is not properly correlated with the local pseudorandom code generator in a receiver, the satellite signal will not be acquired or tracked. Because of the nature of the automatic operation of the system, however, even when the satellite signal is properly correlated there is generally no direct and immediate indication of satellite signal quality or signal acquisition and tracking. Therefore, as more GPS systems are employed, there is a need to provide a simple verification of signal quality and acquisition from the various satellites in order to aid operator use of the system.
Accordingly, the present invention has been developed to provide a verification of satellite signal acquisition and tracking in a GPS system which is inexpensive to implement but reinforces the level of operator confidence in the system.